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H O N O R S C H E M I S T RY – S E M E S T E R 2
Concentration and Molarity
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Concentration of a Solution
𝒂𝒎𝒐𝒖𝒏𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆
𝒂𝒎𝒐𝒖𝒏𝒕 𝒐𝒇 𝒔𝒐𝒍𝒗𝒆𝒏𝒕 𝑶𝑹
𝒂𝒎𝒐𝒖𝒏𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆
𝒂𝒎𝒐𝒖𝒏𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏
More dissolved solute = higher concentration
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Calculating Concentration
Parts per million (ppm) = g solute / 106 g solution
Molarity (M) = mol solute / liter solution
Molality (m) = mol solute / kg solvent
One mol of 8 different elements
1 M CuSO4
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Parts per million (ppm)
Used for VERY small quantities (e.g., lead levels in
drinking water)
Even smaller levels given in parts per billion (ppb)
Mathematical expression: 𝑔𝑟𝑎𝑚𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒
𝑔𝑟𝑎𝑚𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛× 1,000,000 𝑝𝑝𝑚
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Sample Problem – Calculating ppm
A chemical analysis shows that there are 2.2 mg of lead in
5.00 x102 g of a water sample. Convert the measurement
to ppm.
Calculate:
Convert mg to grams of lead
Divide by grams of water
Multiply by 106 PPM
= 4.4 ppm
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Practice
3.0 x 10-4 g of Helium gas are dissolved in 200.0 g of
water. Express this concentration in ppm.
1.5 ppm
A 365.0 g sample of water contains 23 mg Au. How much
gold is present in the sample in ppm?
63 ppm
Additional ppm practice p 461: 2-4, 6-7
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Calculating Concentration – Molality (m)
Used when solvent volume changes with temperature
Calculated as: m = mol solute / kg solvent
Example:
You mix 0.75 mol NaCl with 2.5 kg of distilled water. What is the molality of your solution?
0.30 m NaCl
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Molality (m)
How many moles of sucrose are required to make a
3.5 m solution in 1.75 kg of water?
6.1 mol sucrose
What is the molality of a solution of 52.0 g baking
soda (NaHCO3) in 125 g of water?
4.95 m NaHCO3
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Calculating Molarity
…when given mass of solute and volume of solution
If you are given “mass,” use the
molar mass of the compound to
convert to “moles”
If not already in liters, convert
volume to L
1000 mL = 1000 cm3 = 1 L
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Calculating Concentration – Molarity (M)
Calculated as:
M = mol solute / L solution
Example:
You are told that 0.30 mol KBr are present in a 0.40 L
sample solution. What is the molarity?
0.75 M KBr
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Sample Problem – Calculating Molarity
What is the molarity of a potassium chloride solution
that has a volume of 400.0 mL and contains 85.0 g KCl?
2.85 M
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Practice
Vinegar contains 5.0 g of acetic acid, CH3COOH, in
100.0 mL of solution. Calculate the molarity of acetic acid
in vinegar.
0.83 M acetic acid
A solution of AgNO3 contains 29.66 g of solute in
100.0 mL of solution. What is the molarity of the solution?
1.746 M AgNO3
What mass of KBr is present in 25 mL of 0.85 M solution
of potassium bromide?
2.5 g KBr
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Diluting Solutions
You have a concentrated solution but need a weaker
solution for your lab work.
Molarity makes it easy!
V1M1 = V2M2
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Diluting Solutions
You have a large beaker filled with 10.0 M HCl in your
stock room. Your experiment needs 2.5 L of 0.10 M
HCl. How do you prepare this?
V1M1 = V2M2 V1 = V2M2/M1
0.025 L = 25 mL of 10.0 M acid diluted to 2.5 L
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Diluting Solutions
Given 25 mL of 3.0 M CuSO4•5H20, what is the
molarity of the solution if you dilute it to a final
volume of 1.0 L?
0.075 M CuSO4•5H20
How many mL of 5.50 M NaCl solution do you need
to make 0.450 L of 2.00 M NaCl?
164 mL of 5.5 M NaCl
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Solution Stoichiometry
How much of a chemical solution do you need for the
experiment?
Solution Stoichiometry uses molarity as an additional
conversion factor
Just like regular stoichiometry, only now you have one more
conversion factor available!
M = mol/L
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Solution Stoichiometry
Conversion Review
moles A ↔ moles B
use mole ratio (mol A/ mol B or mol B/mol A)
mass ↔ # mol
use molar mass (g/mol or mol/g)
# mol ↔ volume
use molarity (mol/L or L/mol)
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Sample Problem - Solution Stoichiometry
What volume (in mL) of a 0.500 M solution of copper (II) sulfate is
needed to react with an excess of aluminum to provide 11.0 g of
copper?
1. Write and balance the chemical equation (if not already given)
Unbalanced reaction CuSO4 (aq) + Al (s) Cu (s) + Al2(SO4)3 (aq)
Balanced reaction 3CuSO4 (aq) + 2Al (s) 3Cu(s) + Al2(SO4)3 (aq)
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Sample Problem - Solution Stoichiometry
What volume (in mL) of a 0.500 M solution of copper (II) sulfate is
needed to react with an excess of aluminum to provide 11.0 g of
copper?
3CuSO4 (aq) + 2Al (s) 3Cu(s) + Al2(SO4)3 (aq)
3. Plan your approach
Convert mass Cu to mol Cu with molar mass
Use mole ratio of Cu : CuSO4 to get moles of CuSO4
Convert mol CuSO4 into volume with molarity
= 346 mL CuSO4 solution
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Practice
An excess of zinc metal is added to 125 mL of 0.100 M HCl
solution. What mass of zinc chloride (aq) is formed?
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g)
0.853 g ZnCl2 (aq)
Yellow CdS pigment is prepared by reacting ammonium sulfide
with cadmium nitrate. What mass of CdS can be prepared by
mixing 2.50 L of a 1.25 M Cd(NO3)2 solution with an excess of
(NH4)2S?
Cd(NO3)2 (aq) + (NH4)2S (aq) CdS (s) + 2NH4NO3 (aq)
452 g CdS (s)